What Is an Asteroid?
An asteroid is a lump of rock, metal, or both, that orbits the Sun. Most are small (the size of a car or a building) but some are huge. The biggest, called Ceres, is around 940 km across, which is about half the width of the Moon. Asteroids are the leftover bits from when the planets were being built around 4.6 billion years ago.
- What are they?Bits of rock or metalLeftovers from building the Solar System
- Where do they live?The Asteroid BeltBetween Mars and Jupiter
- How many?MillionsOver 1 million known, perhaps trillions in total
- BiggestCeresapprox. 940 km across, big enough to be a dwarf planet
- Famous dangerChicxulub asteroid10 km across, killed the dinosaurs 66 million years ago
- Visited bySpacecraft like BennuNASA brought a sample home in 2023
How big are famous asteroids?
Diameter in kilometres. Ceres is by far the biggest.
Most asteroids are tiny. Just the few biggest ones in the Asteroid Belt make up almost half of its total mass.
What is an asteroid made of?
Asteroids come in three main types. C-type (carbonaceous) asteroids are dark and rich in carbon, like bits of charcoal. They make up about three quarters of all asteroids. S-type (silicate or "stony") asteroids are made of rocky material with some iron and nickel mixed in. M-type (metallic) asteroids are mostly iron and nickel, the leftover cores of small worlds that broke apart in the early Solar System.
Where do they live?
Most asteroids orbit the Sun in a huge ring called the Asteroid Belt, between Mars and Jupiter. The belt is enormous (approx. 200 million km across) but if you put all the asteroids together they would only make a ball about half the size of the Moon. So even inside the belt, asteroids are usually millions of kilometres apart. The spaceships that visit the belt never have to dodge them.
A second group called the Trojan asteroids shares Jupiter's orbit, sitting in two stable spots ahead of and behind the giant planet. And then there are the Near-Earth Asteroids, which cross or come close to Earth's own orbit. These are the ones we keep an eye on.
Could an asteroid hit Earth?
Yes, and one does almost every day. Small ones the size of a pebble or a basketball burn up harmlessly in the atmosphere as shooting stars. Bigger ones can be a problem. In 2013 an asteroid approx. 20 metres across exploded over the Russian city of Chelyabinsk. The shock wave shattered windows for 80 km around and injured around 1,500 people.
Astronomers now track over 30,000 Near-Earth Asteroids. None of them are on a collision course with Earth in the next 100 years. But just in case, scientists are testing ways to deflect them. In 2022 NASA crashed a spacecraft called DART into a small asteroid moon called Dimorphos, and successfully changed its orbit. It was the first real test of planetary defence.
Deeper dive: the formation of asteroids and missions to study them
Asteroids are the building blocks of the rocky planets that never finished assembling. In the early Solar System, dust and gas swirled around the young Sun. Tiny grains stuck together to form pebbles, pebbles became boulders, and boulders became kilometre-sized objects called planetesimals. Most of these planetesimals eventually joined together to form Mercury, Venus, Earth and Mars. But in the region between Mars and Jupiter, the powerful gravity of Jupiter constantly stirred things up, preventing the planetesimals from joining into a planet. The leftover material became the Asteroid Belt.
Studying asteroids lets us look at almost-unchanged samples of the original material the Solar System was built from. Spacecraft missions have visited several. NASA's NEAR Shoemaker mission landed on Eros in 2001. Japan's Hayabusa mission returned a sample of asteroid Itokawa in 2010. Hayabusa-2 returned samples of Ryugu in 2020, and NASA's OSIRIS-REx returned material from Bennu in 2023. These samples contain organic molecules and water-bearing minerals, supporting the idea that early Earth got at least some of its water and the chemical building blocks of life from asteroid and comet impacts.
The Chicxulub impactor that ended the dinosaur age is known as a K-Pg boundary event. The 200 km wide crater, mostly buried under Mexico's Yucatan Peninsula, was identified in the 1990s by combining oil-industry gravity surveys with thin layers of iridium found in 66-million-year-old rocks worldwide. Iridium is rare on Earth but common in asteroids, providing a chemical signature of the impact. The collision threw enough dust and sulphur into the atmosphere to block sunlight for years, killing off the dinosaurs and around 75% of all species. Modern planetary defence work, including the 2022 DART mission, is partly motivated by avoiding a repeat.
For where most asteroids live, read about the Asteroid Belt. For their icy cousins, see what is a comet.